U.S. Pat. No. 5,516,472 to Laver (“Laver”) is incorporated herein by reference and teaches the use of an extruder in the extrusion of an extruded synthetic wood material, comprised predominantly of cellulosic fibers in a thermoplastic matrix. The synthetic wood material described in Laver is representative of the class of building materials now generally known as wood plastic or cellulosic composites and hereafter referred to as “cellulosic composites,” or “extruded cellulosic composites” or simply “extrudate.” The cellulosic composite is a true composite as opposed to a filled thermoplastic material in that the cellulosic composite possesses properties of the cellulosic fibers, such as stiffness and compressive strength while also possessing properties of the thermoplastic material such as resistance to water, fungal decay, and termite infestation.
Extrusion of this composite results in the production of a material in which the molten thermoplastic is intimately associated with the cellulosic fibers. Although the thermoplastic forms a continuous matrix surrounding the cellulosic filler, the volume ratio of thermoplastic and cellulosic filler is such that the ability of the molten plastic to flow is very limited. As a result the composite will maintain its shape even while the thermoplastic is still in a molten state.
Thermoplastic polymer extrusion, on the other hand, results in the production of a molten, viscous fluid. This fluid may be shaped by passing it through a die in a manner similar to that shown in Laver. However the polymer will not maintain that shape unless it is cooled below its molten state. In fact, it is the nature of the polymer extrudate to swell as it exits the shaping die due to the relaxation of forces imposed on the polymer during the shaping process. Therefore, the desired shape must be maintained until the extrudate has cooled. This is accomplished by drawing the molten polymer through a sizing and cooling die or a series of such dies with a puller device.
A puller device, which is well known by those familiar with polymer extrusion, is a machine that produces traction by means of moving belts, and pulls the extrudate through the sizing devices by grasping the end of the extrudate and mechanically pulling it from the extruder through the sizing devices under pre-designed conditions of time and speed in order to keep the extrudate consistent in shape and size. Reference is made to Laver which describes the use of a puller device in an extrusion process.
Swelling of the extrudate as it exits the shaping die results in a surplus of material between the shaping die and the first sizing/cooling die. The extrudate is drawn down to the desired size and shape. Small variations in volume output from the shaping die can be corrected through this process since some variation in the amount of fluid material is not harmful to the process. Larger variations in output or accumulations of surplus material require some corrective measures. Either volume output from the shaping die or the rate at which material is moved through the sizing/cooling dies by the puller device must be adjusted.
Devices exist that perform these functions in the production of plastic pipe, plastic profiles, and other products made from polymer extrudates. For example, U.S. Pat. No. 4,209,476 to Harris describes one device. This device is designed to maintain the average value of some dimension of a formed and cooled profile close to the desired average value for that dimension. This device works by measuring the volume flow rate of extrudate from the die, quantifying one easily measured dimension of the formed and cooled profile, measuring the length of profile produced in a given time interval, and using a microprocessor to calculate the average value of the desired dimension during the given time interval from the measured volume and length with an adjustment made for swell or shrink as determined from the easily measured dimension. The volume of material passing through the extrusion die in the given time interval must be known in this method. The device presented in Harris measures the volume produced by counting revolutions of a metering or melt pump.
U.S. Pat. No. 4,137,025 to Graves et al describes another control system designed for use in thermoplastic pipe production. This control system measures the wall thickness of a plastic pipe in the early stages of cooling and adjusts puller speed to correct deviations from the desired thickness. The measurement device used in this control system is an ultrasonic measuring device which must be coupled to the surface of the pipe either through direct contact or through a suitable liquid agent. The ultrasonic device rotates around the circumference of the plastic pipe so that measurements of the wall thickness can be taken from multiple locations and averaged. An operator console is provided for the control system however the function is limited to a choice of manual or automatic control. No provision is made for adjustment of the automatic control system while it is functioning.
Another device developed for use in processing plastic profiles is the BETALASER MIKE control device as described in U.S. Pat. No. 6,138,052 to Kristensen. This device uses a laser micrometer to measure a small profile or a small feature of a larger profile. The laser micrometer used by this device has an aperture measuring approximately 4 mm by 15 mm. By placing a small profile within the aperture changes in profile size can be measured. This gives a measurement that can be compared to upper and lower tolerance limits. The principle of the BETALASER MIKE control device is to keep the profile size within tolerance by changing the speed of the puller or haul off device. The BETALASER MIKE control device is claimed to be an improvement over prior devices in the use of an amplified digital signal as opposed to the analog signals used previously. Unlike the devices of Harris and Graves, the BETALASER MIKE control device is represented as a control device for use in the production of foamed polymer profiles in addition to tubing and small unfoamed profiles. In the foam profile application, the profile is generally too large to fit within the aperture of the laser micrometer measuring device. The laser micrometer is mounted on the face of the shaping die so that a small portion or corner of the extrudate lies within the aperture. Changes in the amount of die swell present at the die exit cause the amount of material within the aperture to change. Theoretically, this can be related to volume output of the extruder. Changes in the haul off rate can be made based on the predicted volume output. Obviously this system requires uniform output throughout the cross section of the profile as otherwise changes in the small portion actually measured will not be representative of the profile.
All three of these devices rely on the presence of die swell and on the high melt strength of a polymer extrudate. However, the extrudate described by Laver is not capable of being shaped or sized by these conventional means after it exits the extruder. The cellulosic composite does not swell upon exiting from the shaping die so there is no surplus of material to offset variations in volume output. The cellulosic composite has very low melt strength and is easily torn apart while still in the molten state. As previously noted, the variation in volume output of the cellulosic composite is much greater than that of the polymer extrudate because of the variable properties of the cellulosic fibers. In commercial practice, this cellulosic composite is produced without any downstream sizing and without the use of a puller or haul off device.
While the cellulosic composite can be produced without downstream sizing because of the absence of die swell, it would be advantageous to size the cellulosic composite in some applications. Downstream sizing would improve dimensional tolerance in critical applications and would allow for the introduction of smaller detail in the parts produced. New products might be produced by the application of coatings or of reinforcing fibers downstream from the extruder, something not feasible in a single step process at this time because of the added frictional drag encountered during application.
What is needed is a control device capable of detecting changes in extruder volume output as soon as they begin to occur and responding rapidly to those changes. It would be a further advantage if the control device had some capability of predicting the extent and duration of changes in volume output since this would increase the precision of control.
The rate at which the extrudate is extruded, the rate of expansion of extrudate, and the rate the extrudate moves through the sizing devices must be balanced so that the correct amount of material enters the sizing devices at all times. If the rate of travel of the extrudate through the sizing devices is slower than the rate of extrusion and expansion, then excess material will build up between the extruder and the sizing devices, causing the profile of the extrudate to become deformed. If the rate of travel of the extrudate through the sizing devices is faster than the rate of extrusion and expansion, then the material will not fill the sizing devices also leaving a deformed extrudate. The profile of the extrudate will not have the desired shape or surface characteristics and, in some cases, may be pulled apart due to the lack of material.
The rate at which the extrudate is extruded may vary because of variations in the rate at which material is fed into the extruder or by variations in the feedstock. The rate of expansion may vary due to changes in processing temperatures or due to variability in the amount of gas-producing materials in the feedstock. As noted, the presence of cellulosic fibers increases the variation in both output and expansion. Puller speed can be considered to be constant, but the elasticity of the extrudate can cause variations in rate of travel through the sizing devices as the extrudate will stretch when more force is required to pull the material through the sizing devices and contract when less force is required. The effects of these variations in rate are more dramatic as the production rate increases. Frequent operator intervention is required to keep the system balanced.
It would be advantageous if a device was capable of measuring volume output when expansion was partially completed, could respond rapidly and frequently to measured changes, and further to have some predictive capability regarding the extent and duration of those changes.